Early metaplastic breast carcinoma: A rare tumor with unique clinicopathological features and limited response to neoadjuvant therapy—A single institution experience.

e13183 Background: Metaplastic breast carcinoma (MpBC) is a subtype of breast cancer characterized by both epithelial and mesenchymal features. While MpBC usually presents as a triple-negative breast cancer (TNBC) phenotype, response to neoadjuvant chemotherapy (NAC) is generally poor. As MpBC can express PD-L1, recent studies have investigated the role of PD-L1 inhibitors, such as pembrolizumab, in the treatment of MpBC. We aimed to evaluate patient outcomes of early MpBC, with a focus on clinicopathological and sociodemographic factors that may be predictive of treatment response and recurrence of disease. Methods: A retrospective study was performed to identify all female patients diagnosed with Stage I to Stage III MpBC treated at a safety-net hospital and university affiliated cancer center in Dallas, Texas from January 2013 to October 2024. Results: We identified 55 patients with MpBC with a median age (IQR) of 54, 26% Caucasian, 74% non-Caucasian. The majority of patients were Stage II or Stage III (61%) with 22% having node-positive disease at diagnosis. The majority of patients presented with TNBC (74%). In our cohort, 49 (90.7%) patients received peri-operative chemotherapy and 23 (42.6%) received immunotherapy and chemotherapy. Eight (15%) patients recurred within 10 years of follow up. Patients with younger age (mean age of 46.5 in the recurrence group and 56.5 in non-recurrence; p = 0.02), squamous cell histology (p = 0.02), Caucasian race (p = 0.02) and a Ki-67 of ≥51% were more likely to experience recurrence of MpBC. A residual cancer burden (RCB) score of 2 or 3 was associated with node positive disease (p = 0.02), squamous histology (p = 0.03), and non-TNBC histology (p = 0.02). Addition of immunotherapy with PD-L1 inhibitors did not significantly impact risk of recurrence or RCB (p = 0.46). Conclusions: Overall, this study further demonstrates the aggressive nature of this disease and limited response to systemic therapy and discovered several prognostic factors, such as age, race, and histology on clinical and pathological outcomes.